The invention relates to a switch with a fixed set of contacts and a movable set of contacts, the fixed set of contacts comprising at least one pair of fixed contacts arranged a distance apart and each having an auxiliary contact part and a main contact part and the movable set of contacts comprising at least one auxiliary contact element and a main contact element, which, when the switch is closed, are in contact, under mechanical pre-stress, with the surface of the auxiliary contact part and of the main contact part, respectively, of the pair of fixed contacts and bridge the distance between the free end edges thereof facing one another, the various features being such that when the switch is closed the auxiliary contact element and the main contact element make contact more or less simultaneously with the auxiliary and with the main contact parts, respectively, of the pair of fixed contacts.
A switch of this type is disclosed in European Patent 0 281 622.
The auxiliary contact element, also referred to as the arcing contact, and the main contact element can be sliding contacts which when the switch is closed slide over the fixed contacts into their final position, that is to say the position in which the switch is fully closed. Contact rollers are generally used for closing on large short-circuit currents, for example greater than 30 kA, but contact blades can also be used. However, it is the case that the use of rollers provides a facility for using lighter weight mechanics. In principle, it is necessary only to construct the auxiliary contact elements as rolling contacts. European Patent 0 281 622 describes a switch that is suitable in particular as a current breaker. This current breaker is provided with contact rollers, all contact rollers in a set of contact rollers having the same diameter, but each pair having a different length. The two shortest rollers are positioned closest to the fixed contacts, whilst the longest rollers are furthest removed from the fixed contacts. The fixed contacts are of stepped construction at those edges thereof which face the set of contact rollers, the height of each step corresponding to the difference in length of the contact rollers. The distances between opposite steps are greater than the corresponding longitudinal dimensions of the rolling contacts, so that there is an air gap at every point between the movable contact rollers and fixed contacts when the switch is in the open position.
In the open position the movable rolling contacts are thus separated from the fixed contacts. In order to bring the switch into the closed position, the contact rollers are moved towards the fixed contacts, after which the contact rollers make contact more or less simultaneously with the pair of fixed contacts and then roll, under spring pre-stressing, over the surface of the pair of fixed contacts. In the closed position, the contact rollers are thus in contact, under mechanical pre-stress, with the surface of the pair of fixed contacts and bridge the distance between those free end edges of these fixed contacts which face one another.
The life of the known current breaker is increased by using pairs of contact rollers which as far as possible simultaneously break the contact with the fixed contacts on opening.
From FR-A-2638017 a switch with a fixed set of contacts and a movable set of contacts is known. The fixed set of contacts comprises at least one pair of fixed contacts arranged a distance apart and each having an auxiliary contact part and a main contact part. The movable set of contacts comprises at least one auxiliary contact element and a main contact element, which, when the switch is closed, are in contact under mechanical pre-stress, with the surface of the auxiliary contact part and of the main contact part of the pair of fixed contacts and bridge the distance between the free and edges thereof facing one another.
This known switch is closed and opened in three phases. For this purpose a first pair of movable knifes and a second pair of movable knifes are provided, on which a first set of main contacts and a second set of main contact together with auxiliary contacts are attached. During the closing operation first the movable and fixed auxiliary contacts contact each other, after which the first set of movable main contacts contacts the corresponding fixed contacts and finally the second set of movable main contacts may contact with the corresponding fixed main contacts. A short distance between the second movable main contacts and the second fixed main contact is provided when the first set of movable main contacts begin to engage corresponding fixed main contacts. In FR-A-2638017 a short distance between an auxiliary contact and one of the sets of main contacts are not disclosed nor clearly specified. However, from the figures of FR-A-2638017 one could derive that when the switch is closed, the main contact element makes mechanical and electrical contact with the main contact part of the fixed contacts only after the auxiliary contact element has travelled a distance over the surface of the auxiliary contact part of the fixed contacts.
The aim of this invention is to provide a switch of the type mentioned in the preamble with which the life of the switch is lengthened without additional contacts.
Said aim is achieved according to the invention in that said travelled distance is at least 2 mm
In FR-A-2638017 absolutely no indication is given that a minimum leading distance of the auxiliary contact is of essential importance when the swith is closed and absolutely not a specified minimum distance of 2 mm. The measure known per se of leading is primary used for preventing contact burning when the switch is switched out. By recognizing and using the minimal distance of the 2 mm it is possible to prolong the life of the switch without adding extra contacts. In the switch of FR-A-2638017 two set of main contacts are needed. Herein after on page 8, lines 1 and 2 it is indicated that the switch according to the present invention is suitable for closing on very high short circuit currents. Under these conditions it is just important that the condition of abovementioned minimal distance is met because just under these circumstances longer vibration time should be taken into account.
By the above-mentioned distance of 2 mm a distinct separation is obtained between the time when the movable auxiliary contact element comes into contact with the fixed contacts and the time when the main contact element comes into contact with the fixed contacts. All switching operations are now, as it were, carried out by the auxiliary contact element and virtually not by the main contact element and all closing and opening phenomena which occur during switching occur only at the auxiliary contact element and the associated auxiliary contact part of the fixed contacts. The consequence of this is that the auxiliary contact element is damaged, whilst the main contact element remains undamaged. Since the main contact element carries the major proportion of the current it is important that this main contact element remains undamaged, even when closing on a short-circuit current. An undamaged main contact element then also contributes significantly to a longer life of the switch. The phenomenon of switching with the arcing contacts only and not with the main contacts has already been known for some time from power switch technology. However, in the latter case it is not xe2x80x9cslidingxe2x80x9d contacts but xe2x80x9cbuttxe2x80x9d contacts that are concerned, as a result of which a completely different embodiment has been produced with the associated different problems. In the case of the present contact construction the contacts are pressed onto one another on short circuiting by the electromagnetic forces that arise. In the case of a contact construction with butt contacts these forces have the effect of reducing the contact pressure.
The switch according to the invention is particularly suitable as a load switch, which in general is connected in series to a safety fuse for breaking and limiting the short-circuit current and in particular is connected in series to a current-breaking and/or current limiting device. The main contact element according to the invention switches only when the short-circuit current has been clipped or limited.
In a first embodiment the movable main and auxiliary contacts are integrated in one contact.
In a second embodiment of the invention the main contact parts of the fixed contacts are located in a first plane and the auxiliary contact parts of the fixed contacts are located in a second plane that is some distance away from and parallel to the first plane and the plane of movement of the main contact element is parallel to the first plane a small distance away and the plane of movement of the auxiliary contact element is parallel to the second plane a small distance away.
In a third embodiment of the invention the main contact parts and auxiliary contact parts of the fixed contacts are located in one plane and the planes of movement of the main contact elements and the auxiliary contact elements are coincident, whilst these planes correspond to the plane of the main and auxiliary contact parts of the fixed contacts, the lengths of the auxiliary and main contact elements differing from one another in the bridging direction and the spacing of the fixed contacts in the switching direction being reduced stepwise.
It is possible to use more auxiliary contact elements one after the other in the plane of movement with more steps in the fixed contacts, the auxiliary contact elements making contact more or less simultaneously with the associated auxiliary contact parts of the fixed contacts when the switch is closed, in order to distribute the current over the auxiliary contact elements at the start of contact.
In European Patent 0 281 622 it is stated that a reduction in the repellent forces during the closing operation (repellent forces are electromagnetic forces which arise when a high current passes between two contacts brought into contact with one another) is achieved by the use of the parallel rollers. The repellent forces referred to here are forces which arise perpendicularly to the surface of the fixed contact. Another force which arises is the repulsion force. In principle, the repulsion force has the same physical background but is oriented parallel to the surface of the fixed contact. In this invention use is made of the fact that the repellent forces reduce when there are more parallel contacts, but then only if the contacts are fully engaged, for example when short-circuit currents pass through. Repulsion forces, however, arise only when a switch is closed and directly oppose the mechanics and do so significantly in the case of short circuits, as a result of which the mechanics have to be of heavier construction than is necessary.
In a further development of the second embodiment of the invention the repulsion force when the switch is closed is reduced in that a slot is made between the auxiliary contact part and the main contact part of the fixed contacts.
By making the slot in the fixed contacts the course of the current path therein is influenced in such a way that the length of the part of the current path that runs parallel to the direction of movement of the movable auxiliary contact elements in the auxiliary contact part of the fixed contacts is reduced. This reduced length results in a lower repulsion force, which is important in particular when closing on a short circuit and any reduction that can be achieved in the repulsion force translates directly back into lighter weight mechanics, which is desirable for many reasons.
It is pointed out that in German Patent Application DE 32 23 654 A1 laid open for inspection a recess is made in a fixed contact, but this recess does not run through the entire thickness of the fixed contact and thus does not have the effect of the slot according to the invention.
Furthermore, it is also pointed out that a slot in the fixed contact is known per se from German Patent DE 3 504 605 C2, but the function of this slot is to lengthen the arc when opening the switch and this cannot reduce the repulsion force of the moving contacts.
Embodiments of the slot that are preferably to be used are described in the dependent claims.
In one embodiment of the invention the auxiliary contact part of the fixed contact is narrowed to an L-shape, the free end of the leg extending in the opposing direction to the direction of movement for closing the movable auxiliary contact element and forming the track for the element. Preferably, a projection is formed on the free end of the leg, which projection extends from the track in a direction away from the free space between the fixed contacts, the free end of the movable auxiliary contact element not coming into contact with the projection when the switch is closed.
The advantage of this projection is that the foot of the arc that is produced when the switch is opened will move rapidly towards both the point of the movable auxiliary contact element and the point of the projection, as a result of which any damage will manifest itself there. These two damaged parts will, however, not come into contact with one another or with other contact parts when the switch is closed, which means that the closing and opening characteristics as well as the current-carrying characteristics are also not changed as a result of any damage as a consequence of the arc, as a result of which the life of the switch is prolonged. If this projection were not present, the arc would damage the rising edges of the contact, which has an adverse effect on the closing characteristics. This adverse effect is observed in particular when closing on short circuits and the durability of the switch will also deteriorate substantially at this point.
Embodiments of the projection which are preferably to be used and the associated part of the auxiliary contact element are described in the further dependent claims.